A fiber-reinforced thermoplastic resin composite in which a continuous reinforcing fiber bundle is impregnated with a thermostatic resin is generally called a prepreg, and is widely used as a base to be molded into the members of motor vehicles and airplanes, general industrial materials, and, in addition, sporting and leisure applications such as golf clubs.
Since a molded article obtained by molding said prepreg is required to have surface appearance quality, mechanical properties, etc., it is necessary to sufficiently impregnate the reinforcing fiber bundle with a resin, for decreasing voids as far as possible. For this reason, a prepreg in which the reinforcing fiber bundle is virtually perfectly impregnated with the thermoplastic resin, a so-called perfect impregnation type prepreg is preferably used.
In this case, impregnation refers to a state where the thermoplastic resin permeates among single fibers of the reinforcing fiber bundle substantially without any clearance.
However, though this perfect impregnation type prepreg allows the voids in the molded article to be decreased, it is poor in the handling capability (hereinafter called drapability) experienced when it is molded for being shaped according to a complicatedly shaped mold since it is rigid, and it is greatly limited in the degree of shaping freedom of the final product. Furthermore, in general, in order to make the reinforcing fiber bundle impregnated with the thermoplastic resin sufficiently inside, a step of, for example, forcibly “stroking” the composite comprising the molten resin and the reinforcing fiber bundle is necessary, and the reinforcing fiber bundle becomes, for example, fluffy, to cause such problems as lowering the appearance quality and mechanical properties of the obtained molded article. Moreover, such impregnation may require a long time as the case may be, and the productivity of the molded article is another problem.
As described above, in view of the impregnability and drapability to be maintained at high levels, the perfect impregnation type prepreg has a technical limit.
So, in recent years, prepregs of a comingled form and a discontinuous comingled form have been developed.
In this case, a comingled form refers to a composite form in which a continuous thermoplastic resin is made to exist as fibers inside a continuous reinforcing fiber bundle. Furthermore, a discontinuous comingled form refers to a composite form in which a discontinuous thermoplastic resin is made to exist as fibers in a continuous reinforcing fiber bundle.
For example, JP60-209033A discloses a method for producing a prepreg of a comingled form comprising a continuous reinforcing fiber bundle and a continuous thermoplastic resin fiber bundle. This form is excellent in drapability, since a prepreg per se is already impregnated with a thermoplastic resin, and furthermore, since the reinforcing fiber bundle and the thermoplastic resin are disposed nearby, the impregnability is also good. However, when the prepreg is carried or shaped according to a mold, it can happen that the reinforcing fiber bundle and the thermoplastic resin fibers are separated, and a preliminary step of spinning the thermoplastic resin into a multifilament is needed. So, after all, the comingled form cannot be industrially used in view of productivity and handling convenience.
Moreover, JP03-47713A discloses a process for producing a prepreg of a discontinuous comingled form, comprising the steps of placing a sheet comprising short discontinuous thermoplastic resin fibers cut to a length of 20 mm to 200 mm oriented at random on a continuous reinforcing fiber bundle, and intermingling forcibly using such an intermingling method as a water jet. This form is also good in the impregnability during molding. Furthermore, since the form is kept because of perfect interminglement, the problem that the reinforcing fiber bundle the thermoplastic resin fibers are separated does not arise. However, since the thermoplastic resin fibers are disposed as short fibers, the prepreg becomes bulky and has such a problem of drapability that it cannot be shaped depending on the shape of the mold. Furthermore, since such a forced intermingling method as a water jet is used, it can happen that the reinforcing fibers are broken or curved, and there arises such a problem that the molded article declines in surface appearance quality, mechanical properties, etc. Moreover, a step of spinning a thermoplastic resin into a multifilament and a step of cutting it into short fibers using a cutter or the like are needed additionally. As a result, like a comingled form, this form cannot be industrially used either in view of productivity.
As described above, no prepreg satisfying impregnability, drapability, handling convenience and productivity at high levels has been found yet.